Replace offline basemap pipeline with Planetiler, unify online/offline styling

Offline tiles were built from Protomaps' own basemap schema, needing a
completely separate hand-tuned theme (protomaps-themes-base + OFFLINE_THEME)
from the online Liberty style's ONLINE_STYLE_PATCH — the two could never be
made to match exactly no matter how much either side got tuned (today's
session found and fixed several mismatches: building opacity, park
boundaries, road casings, road widths).

Offline.pmtiles is now built with Planetiler's default OpenMapTiles profile
against Geofabrik per-country OSM extracts (osmium-merged when more than one
country is selected), the same vector schema Liberty already expects.
offlineStyle() now clones the same Liberty JSON (vendored as a local
snapshot for reliability with no internet) and applies the identical
ONLINE_STYLE_PATCH, just repointed at local PMTiles/glyphs/sprite — one
palette instead of two, correct on both sides by construction.

Also vendors Liberty's icon sprite for the first time (offline had none
before), adds Java/osmium-tool/planetiler.jar to the Docker image, and
removes the now-unused protomaps-themes-base dependency and the
theme-comparison debug page that found today's bugs.

app.rb

@@ -260,10 +260,9 @@ module Maptool
     end
 
     # Regenerates offline.pmtiles for the currently selected regions (see
-    # POST /api/offline-regions above) by extracting them from Protomaps'
-    # own daily full-planet basemap build — see
-    # lib/maptool/pmtiles_builder.rb. Runs in the background; poll
-    # build-status below for progress.
+    # POST /api/offline-regions above) by downloading each from Geofabrik
+    # and tiling them with Planetiler — see lib/maptool/pmtiles_builder.rb.
+    # Runs in the background; poll build-status below for progress.
     post "/api/offline-tiles/build" do
       require_login!
       begin